1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an inverter for a backlight unit and an LCD module using the same.
2. Discussion of the Related Art
In general, an active matrix (AM) type LCD device displays moving images using thin film transistors (hereinafter, referred to as “TFTs”) as switching elements. In comparison with a cathode ray tube (CRT), the LCD device can have a smaller size. Thus, the LCD device may be used in a wide variety of applications, such as personal computers, notebook computers, office equipment, and mobile phones.
The LCD device is not a self-lighting emission device. In this respect, the LCD device necessarily requires a light source, for example, a backlight unit, which is operated by an inverter. Among various components provided in the LCD device, the backlight unit consumes the most power.
The inverter that drives the backlight unit is generally divided into a DC/DC inverter and a DC/AC inverter. In this case, the DC/DC inverter generates a DC voltage by a pulse width modulation (PWM) method. Also, the DC/AC inverter converts the DC voltage supplied from the DC/DC converter into a high AC voltage having a level suitable for driving a lamp. However, the inverter for the LCD device of the related art consumes a large amount of power due to the low efficiency of a transformer included in the DC/AC inverter.
FIG. 1 is an exploded perspective view of a related art LCD device.
As shown in FIG. 1, the related art LCD device includes an LCD panel 30 to display images and a backlight unit. The backlight unit includes a fluorescent lamp 31 to emit light, a U-shaped lamp housing 32 to surround the fluorescent lamp 31, a light-diffusion sheet 35, a first prism sheet 37, a second prism sheet 36 and a protective sheet 38 provided in sequence below the LCD panel 30. The backlight further includes a light-guiding plate 33, a reflective sheet 34, and main support 39 to support and fix the LCD panel 30 and the backlight unit.
A display area (A) of LCD panel 30 is illuminated with the light emitted from the backlight unit. Although not shown, the display area (A) of LCD panel 30 is comprised of two transparent substrates bonded to each other, and a liquid crystal layer formed by providing liquid crystal material in an inner space between the two transparent substrates. Further, two polarizing plates are adhered to respective outer surfaces of the two substrates. A driving circuit 40 is provided to drive the display area (A).
The backlight unit operates as follows. When the fluorescent lamp 31 provided at one end of the light-guiding plate 33 is turned on, the light emitted from the fluorescent lamp 31 is reflected by the lamp housing 32. Then, the reflected light is transmitted to an end of light-guiding plate 33, in which the fluorescent lamp 31 is not provided, through a cross section of light-guiding plate 33. As a result, the light is diffused to the entire surface of light-guiding plate 33, and the display area (A) of LCD panel is illuminated by the light using the light-diffusion plate 35.
In the related art LCD device, thin film transistors formed in the LCD panel control respective pixels according to a signal provided by the driving circuit 40 so that the light for the display area (A) of LCD panel is selectively transmitted. By controlling light transmission through the respective pixels, the image is displayed on the display area (A) of LCD panel.
FIG. 2 is an exploded perspective view of a transformer for a lamp-driving circuit of an LCD device according to the related art. FIG. 3 is a perspective view of the transformer shown in FIG. 2.
As shown in FIGS. 2 and 3, the transformer for the inverter of the LCD device comprises a bobbin 1, on which coil 2 is wound, provided with partitions 1a at fixed intervals, and ferrite cores 4a and 4b inserted into the center of bobbin 1. Here, the bobbin 1 having the partitions 1a is formed of a plastic material. The ferrite cores 4a and 4b are formed by mixing minutely powdered iron oxide and manganese together, thereby being able to guide a magnetic flux.
Each of the ferrite cores 4a and 4b is formed in the shape of an “E” wherein the central part of the “E”-shaped ferrite core penetrates the center of bobbin 1. The sidewalls of ferrite cores 4a and 4b surround the sides of bobbin 1 having the coil 2 wound thereon.
Based on a predetermined winding ratio, the winding number of coil 2 at a first side is different from the winding number of coil 2 at a second side. A current flows through the coil 2.
At both ends of the bobbin 1, there are lead pins 3. In detail, the coil 2 is wound on each grooved coil-winding part between the partitions 1a. Then, a tape is wound on the coil-winding part of bobbin 1 having the coil 2 wound thereon. After assembling the ferrite cores 4a and 4b into the bobbin 1, the ferrite cores 4a and 4b are bound up with an adhesive tape.
The coil-winding parts having the coil 2 wound thereon are divided by the sidewalls of bobbin 1 and partitions 1a of bobbin 1. In this case, the coil-winding parts include a low tension part, a medium tension part, and a high tension part.
FIG. 4 is a cross sectional view illustrating a lower part of the LCD module according to the related art including an inverter, a bottom cover, and a cover shield.
As shown in FIG. 4, an inverter 170 is positioned at a predetermined interval from a rear surface of bottom cover 150. Also, a cover shield 160 covers the inverter 170. Here, the inverter 170 comprises a printed circuit board (PCB) 190, a transformer 180, and high-tension and low-tension patterns 182. There is a connector (not shown) in the PCB 190. Through the connector of PCB 190, the inverter 170 is connected to a fluorescent lamp (not shown) so that power is supplied to the fluorescent lamp. Also, a plurality of integrated circuits (ICs) are provided in the PCB 190, thereby controlling the operation of the inverter 170.
The transformer 180 comprises a core to guide a magnetic flux; a bobbin (not shown) having first and second coils (not shown) wound thereon; and first and second lead pins 183 and 184 respectively connected to the first and second coils at both sides of the bobbin (not shown). The first and second lead pins 183 and 184 are soldered on the PCB 190. As the transformer 180 is supplied with DC voltage outputted from the PCB 190, the transformer 180 generates AC voltage to drive the lamp.
The total thickness of inverter 170 is measured by adding the thickness of PCB 190 and the thickness of transformer 180 together. In this case, the transformer 180 is thicker than the PCB 190. Thus, the total thickness of inverter 170 is determined by the thickness of transformer 180. The transformer 180 has a considerable thickness so that the inverter 170 also has a considerable thickness. The rear surface of the cover shield 160 to cover the inverter 170 is formed at a predetermined height from the rear surface of PCB 190 to the upper surface of transformer 180. That is, the transformer 180 is positioned at a predetermined interval from the cover shield 160, thereby defining an insulation distance. To obtain an insulation distance between the bottom cover 150 and the transformer 180, the transformer 180 is positioned at a predetermined interval from the bottom cover 150. Accordingly, the thickness of LCD module is increased due to the thickness of the transformer 180.
If a high-tension voltage is supplied to a high-luminance lamp, the transformer 180 of the inverter 170 must be thick. In the related art structure, the total thickness of the LCD module is mostly due to the thickness of the transformer 180. By supplying the high-tension voltage from the inverter 170, the LCD module must be thicker.
However, the related art inverter has the following disadvantages. First, it is difficult to decrease the total thickness of the inverter due to the thickness of the transformer used as a voltage-boosting means. In an LCD module for a television, the fluorescent lamps are arranged parallel to each other so that the cost of the inverter is decreased. Thus, the thickness of the transformer is increased to realize the high capacity so that the thickness of inverter as well as the total thickness of the LCD module is also increased.